1. Field of the Invention
The present invention relates to an optical recording medium in which a substrate, a recording film, and a protective film are layered upon one another, and in which recording signals can be reproduced optically from the protective film side.
2. Description of the Related Art
Ordinary optical recording media have a structure in which a recording film (reflecting film) 3 and a protective film 4 made of an organic resin of about 10 .mu.m thickness for protecting the recording film 3 are formed on a transparent substrate 2 made for example of polycarbonate, as shown in FIG. 2. The recording film 3 is constituted of pits (convex and concave portions) 3a that are formed on the surface of the substrate 2 and correspond to recording signals, and a reflecting film 3b made of a metal vapor deposition film of, for example, aluminum formed on these pits 3a. To reproduce recording signals recorded on an optical recording medium 1 with this structure, the recording medium 1 is irradiated with laser light 5 from the side of the substrate 2.
With regard to film hardness and durability, it is preferable to use a UV-curing resin for the material constituting the protective film of such an optical recording medium. Examples of known protective films include a protective film made of a so-called radical polymerizable material (Japanese Patent Application Laid-Open No. H7-70472), in which a resin component is polymerized/cured using a radical polymerization catalyst, and a protective film made of a so-called cationic polymerizable material (Japanese Patent Application Laid-Open No. S59-227045), in which a resin component is polymerized/cured using a cationic polymerization catalyst.
Recently, such optical recording media are used not only as audio media, but also as video media, for which the reproduction of vivid video information over long periods of time is required. Therefore, there is a strong demand for optical recording media with higher recording density and better reproduction characteristics.
As one of the related technologies that attempt to respond to these demands, it has been suggested to optically reproduce the recording signals by irradiating the recording medium with laser light not from the transparent substrate side, but from the side of an optically transmissive protective film (Japanese Patent Application Laid-Open No. H9-147417). According to this technology, the thickness of the optically transmissive protective film is set to 0.5 mm (500 .mu.m) or less, preferably to about 0.1 mm (100 .mu.m) or less, and on the other hand, the thickness of the substrate is, as conventional, about 1.2 mm. When the protective film is relatively thin compared to the substrate as in this technology, the influence of tilts in the surface (irregularities in the coating) or tilts in the optical recording medium itself can be abated. Therefore, when the recording signals are reproduced by irradiating the recording film with laser light from the side of an optically transmissive protective film, it is reasonable to expect that recording signals that have been recorded at a high density in the recording film can be reproduced better than when the reproduction is carried out from the side of a transparent substrate.
However, although the protective film is thinner than the substrate in the optical recording medium disclosed in Japanese Patent Application Laid-Open No. H9-147417, in which the recording signals are reproduced optically from the protective film side, it is much (about 50 times) thicker than the protective film in conventional optical recording media (ca. 10 .mu.m).
When such a thick protective film is made using a conventional UV-curing radical polymerizable protective film material, the protective film shows curing shrinkage after the UV light irradiation, and there is the problem of so-called "warping of the optical recording medium". On the other hand, when the optical recording medium uses a protective film made of a cationic polymer material, the influence of curing shrinkage is small, so that the problem of "warping of the optical recording medium" hardly occurs, but there is the problem of "corrosion of the recording film".